Quantitation of the O(2)-Dependent, CO(2)-Reversible Component of the Postillumination CO(2) Exchange Transient in Tobacco and Maize Leaves

The postillumination transient of CO₂ exchange and its relation to photorespiration has been examined in leaf discs from tobacco (Nicotiana tabacum) and maize (Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O₂ in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO₂ exchange equivalents (i.e. nanomoles CO₂) versus time were derived from the analyzer responses of individual transients. In tobacco (C₃), subtraction of the time course of cumulative CO₂ exchange under photorespiratory conditions (21 or 43% O₂) from that obtained under nonphotorespiratory conditions (1% O₂) revealed the presence of an O₂-dependent and CO₂-reversible component within the first 60 seconds following darkening. This component was absent in maize (C₄) and at low external O₂:CO₂ ratios (i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O₂. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO₂ evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.